A LED projector has been proposed that has a light source device that employs a light-emitting diode (LED) as a light source. Compared to a discharge lamp, an LED, which is a solid-state light source, has the advantages of being mercury-free, capable of quick-start/quick-off lighting operation, and is highly durable. An LED projector of this type of the prior art is made up of a light source device, illumination optics into which light from the light source device is irradiated, a light valve having a liquid crystal display panel into which light from the illumination optics is irradiated, and projection optics for projecting light from the light valve onto a projection surface.
In order to increase the luminance of a projected image in this type of LED projector, it is desirable that all possible measures be taken to limit optical loss in the optical path from the light source device to the projection surface.
The two chief causes of optical loss that can be considered are as described below.
The first cause is polarization loss that arises from the polarization dependence of a liquid crystal display panel or dichroic prism.
The second cause is the occurrence of light that diverges and does not enter each optical element arranged on the optical path, i.e., optical loss resulting from eclipse in each optical element. This occurs because light from a light source device cannot be used as projection light unless the product of the angle of radiation (solid angle) of emitted light from optical elements belonging to the light source device and the light emission area (etendue) is made equal to or lower than the value of the product of the area of the display element and the acceptance angle (solid angle) that is determined by the f-number of the projection lens.
As one countermeasure for solving the above-described problems relating to optical loss, the use of an optical element that has a hologram has been proposed with the object of generating light that is irradiated into a liquid crystal display element.
As one example of an optical element that uses a hologram, Patent Document 1 discloses optical element 101 that is provided with: light guide body 103 into which light from a fluorescent tube as a light source is irradiated; and hologram layer 104 that diffracts light from this light guide body 103, as shown in FIG. 1. In this optical element 101, light that is irradiated into hologram layer 104 at an angle φ formed by the light from light guide body 103 and the interference fringe of hologram layer 104 is diffracted in light guide body 103 at an angle of emission α from hologram layer 104.
As another example of an optical element that employs a hologram, Patent Document 2 discloses optical element 102 that is provided with light guide body 107 into which is irradiated light from LED 106 as the light source, diffusion hologram layer 108 that diffracts light from this light guide body 107, polarization-conversion layer 109 that confers a predetermined phase difference to mutually orthogonal polarization components belonging to the transmitted light, and polarization-separation layer 110 that emits only light of linearly polarized light that has a polarization component of a specific direction from this polarization conversion layer 109, as shown in FIG. 2.